The prior art illustrated in FIG. 1 is related to a U.S. Pat. No. 6,594,844B2. As illustrated, a measurement 1 is applied on an instrument at the bottom of the automatic vacuum cleaner to emit at a certain inclination through an infrared ray 10 to a ground 12; and another infrared ray 14 receives the reflection. Meanwhile, the distances respectively among the infrared ray 10, the infrared ray 14 and the ground 12 are measured from a first path 16 and a second path 18. Once the distances respectively among the first path 16, the second path 18 and the ground changes, it indicates that the distances respectively among the infrared ray 10, the infrared ray 14 and the ground 12 also change accordingly so as to notify the automatic vacuum cleaner to take turns to avoid falling from the higher level on the ground.
However, the prior art is found with the following disadvantages. Firstly, a technical bottleneck exists about the range of detection distance by both of the infrared rays 10 and 14 at a certain inclination. The detection sensitivity becomes poor once a certain range is challenged. Secondly, should the eradiation of the infrared ray be adjusted for smaller power, it may effectively shorten up the detection distance between the infrared ray and the ground. However, the inaccurate measurement of the distance may still happen in case of a dark ground or a ground that pays back poor reflection.